I had a smaller amp doing this. The problem was defective filter caps. They looked perfectly fine but were open internally.

Connect the scope's ground clip (the one that attaches to the end of the probe) to the main ground terminal amp and monitor the ripple on the B+ terminal. If you see any high frequency hash, the B+ filter caps are likely defective. If it's clean, do the same test (monitor the secondary rail voltage) with the scope grounded to the secondary center tap. Make sure that the tap is at ground potential before connecting the ground lead for the probe.